Method and apparatus for radiofrequency ablation with increased depth and/or decreased volume of ablated tissue

ABSTRACT

A method of ablating a tissue site includes at least two stages. A first stage involves conducting bipolar ablation between a first pair of electrodes situated in an opposing arrangement on opposing sides of the tissue site to form a pair of opposing first stage ablation regions extending from respective sides of the tissue towards the center. A second stage involves conducting bipolar ablation between a second pair of electrodes situated in a diametrical arrangement with respect to the first stage ablation regions, which forms a second stage ablation region intermediate the pair of first stage ablation regions. The second stage completes the ablation through the entire depth of the tissue site. Since the overall process can accommodate incomplete ablation during the first stage, lower power, reduced ablation times or both may be used during the first stage, avoiding overheating and with a decrease in ablated tissue volume.

This application is a continuation of U.S. application Ser. No.12/428,173, filed 22 Apr. 2009, now ______, which is hereby incorporatedby reference as though fully set forth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The present disclosure relates generally to ablation systems, and moreparticularly, to a method and apparatus for radiofrequency ablation withincreased depth and/or decreased volume of ablated tissue.

b. Background Art

It is known to deliver radiofrequency (RF) energy to a desired targetarea through an electrode assembly to ablate tissue at the target site.RF ablation may generate significant heat, which if not controlled canresult, generally, in undesired or excessive tissue damage. It isparticularly difficult to achieve successful RF ablation of live,relatively thick biological tissue, for a number of reasons. Thecross-sectional profile (Le., depth) of ablation is often too shallow,in part because most of the Ohmic heat is generated near the RFelectrodes, i.e., at the surface of the tissue. Furthermore, the rate atwhich heat diffuses deep into the tissue layer(s) is extremely slow, andmay be counteracted by the cooling effects due to blood perfusion, forexample, during epicardial ablation. As the tissue surrounding theelectrodes is being ablated, its Ohmic resistivity increases. Thisincrease in resistivity further exacerbates the problem of non-uniformheating, because at a given current density, the heating intensityexperienced by the tissue is proportional to its then-existing Ohmicresistivity.

Moreover, the goals of thick layer RF cardiac ablation are often atodds. It is often necessary to ablate all the way through the thicknessof the tissue, while at the same time avoiding tissue overheating andminimizing the volume of ablated tissue. However, the thicker the tissuelayer, the less possible it is to achieve adequate ablation through itsthickness without overheating the tissue near the RF electrodes (i.e.,damaging the tissue in an unacceptable way) and without ablating anundesirable excess volume of tissue.

There are a number of two-electrode or multi-electrode RF ablatingdevices known in the art, for example as seen by reference to U.S. Pat.No. 7,241,292 entitled CARDIAC ABLATION DEVICE WITH MOVABLE HINGE issuedto Hooven. Hooven discloses a transmural RF ablation device. However,Hooven does not address the problems of inadequate ablation depth orexcessive ablated tissue volume described above, particularly for arelatively thick biological tissue layer.

There is therefore a need to minimize or eliminate one or more of theproblems set forth above.

BRIEF SUN/MARY OF THE INVENTION

One advantage of the methods and apparatus described, depicted andclaimed herein relates to increased depth of RF ablation lesions whileat the same time reducing the overall volume of ablated tissue ascompared to known approaches.

This disclosure is directed to methods of ablating tissue site betweendiverse electrode pairs and the methods include at least two discreteablation procedure stages. During a first stage, an embodiment of theinventive method involves conducting radiofrequency (RF) energy (e.g.,bipolar or unipolar RF energy) ablation between a first pair ofelectrodes situated in an opposing arrangement on opposing sides of thetissue site. The first stage ablation forms a pair of opposing firststage ablation regions extending from respective sides of the tissuetowards the center of the site. However, the first stage may notcompletely ablate the site through its depth. During a second stage inone form of the invention, the methods involve conducting bipolarablation but between a second pair of electrodes situated in adiametrical arrangement with respect to the first stage ablationregions. The second stage ablation forms a second stage ablation regionintermediate the first stage ablation regions, effectively bridging thepair of first stage ablation regions. To the extent an un-ablatedchannel, region or gap exists between the pair of opposing ablationregions after the first stage, the second stage completes the ablationthrough the entire depth of the tissue site. Since the invention canaccommodate incomplete ablation or non-transmural ablation lesions(i.e., a lesion due to ablation that does not penetrate the entire depthof the tissue site) during the first stage, lower power, reducedablation times or both may be used during such first stage, with aresultant decrease in ablated tissue volume. The second stage completesthe ablation process throughout the tissue depth, providing increaseddepth. As note, unipolar or bipolar electrical configurations can beused, or combinations of each type of energy delivery.

A system for RF ablation according to the invention is also presented.In such a system a pair of opposing substantially resilient elongatemembers couple electrodes for RF energy delivery therebetween. Theelongate members can comprise clamping members actuated via a pivot sucha scissor-like unit or a pair of members slideably coupled so that oneis fixed and the other moveable toward the fixed member. Suitableelongate conductors, switches couple the electrodes to an RF generator.In one form the members are designed and fabricated of biocompatiblematerial and of dimension providing ease of pericardial access via, forexample, a sub-xiophoid incision to access one or more pulmonary veinsof a subject. In another form, the elongate members comprise at leasttwo pairs of opposing members so that both pairs can engage a targettissue site in a compressive state and at least one can be relaxed in anon-compressive state thereby not engaging the target tissue site. Inanother form an insulative sheath can be advanced over one or more ofthe electrodes to selectively insulate same from the target tissue siteduring subsequent ablation procedures. The electrodes can be coupled toonly an RF generator or one or more electrodes can also couple to acardiac pacing circuit and/or a cardiac sensing circuit or impedancemeasuring circuit according to various forms of the invention. Thepacing and/or sensing circuitry can be used to confirm transmurality ofa particular lesion set (e.g., a continuous transmural lesion around aplurality of pulmonary veins) whether or not the set was produced by asystem according to the foregoing or in part by another apparatus. Thatis, pacing or sensing “above” the lesion set (above natural sinusrhythm) should not conduct or capture the myocardium thus indicatingcontinuity of the lesion set. Likewise, sensing above the lesion setshould not detect cardiac activity (or at least not at typicalamplitude(s)).

These and other benefits, features, and capabilities are providedaccording to the structures, systems, and methods depicted, describedand claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block and diagrammatic view of an apparatus for RF ablationof a tissue layer, in an embodiment.

FIG. 2 is a flow chart diagram of a method of ablating the tissue siteas shown in FIG. 1.

FIG. 3 is a cross-sectional view of a tissue site shown, in a firstembodiment, in an initial un-ablated condition.

FIG. 4 is a timing diagram of voltage waveforms produced incounter-phase.

FIGS. 5-6 are cross-sectional views of the tissue site of FIG. 4, shownin the first and second stages of ablation, respectively.

FIG. 7 is a timing diagram showing a variation of the embodiment ofFIGS. 5-6 that involves moveable electrodes, where the second stage issub-divided into first and second sub-stages.

FIGS. 8-9 are cross-sectional views of a tissue site in a furtherembodiment involving moveable electrodes and an optional mechanicalclamp.

FIG. 10 is a side view of a first, scissors-type apparatus embodimentfor performing the ablation method of FIG. 2.

FIG. 11 is a side view of a second, slide-type apparatus embodiment forperforming the ablation method of FIG. 2.

FIG. 12-13 are top and side views, respectively, of a third, three-armscissors-type apparatus embodiment for performing the ablation method ofFIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 1 is asimplified Hock and diagrammatic view of a system 10 for conducting RFablation of a live, biological tissue site 12. The system 10 isconfigured to increase the depth of lesions created through RF ablationas well as reduce the volume of ablated tissue for a given tissuethickness. The method divides the ablation process into two main stages,which, without loss of generality, will be described in connection witha number of bipolar ablation embodiments. Although there are manypossible uses of the present invention, one in particular may involveepicardial ablation, such as, for example, as therapy in connection withthe treatment of atrial fibrillation. In this regard, the tissue site 12may comprise either a single layer of tissue (e.g., tissue wall), or mayalternately comprise a pinched or clamped pair of tissue layers such asdescribed and illustrated in U.S. Pat. No. 7,231,292 to Hooven (noted inthe Background), herein incorporated by reference in its entirety. Ofcourse, other applications and purposes are possible, as understood inthe art.

With continued reference to FIG. 1, the system 10 may include an energysource, such as an RF ablation generator 14. The RF ablation generator14 may serve to facilitate the operation of an ablation procedure andmay involve monitoring any number of chosen variables (e.g., temperatureof the ablation electrode, ablation energy, duration) and providing therequisite energy source to electrodes, shown in block form as electrodeblocks 16 ₁ and 16 ₂.

The ablation generator 14 may include a main controller 18, a userinterface 20, an RF power source 22 and optionally an impedancedetection circuit 24. The main controller 18 may be configured tocontrol the operation of the RF power source 22 in accordance withvarious inputs (e.g., power setting, ablation duration (time), etc.)received from a user thereof through the user interface 20. Thecontroller 18 may be further configured interact with the impedancedetection circuit 24 to obtain data indicative of an impedance valuemeasured through or using one or more of the electrodes within electrodeblocks 16 ₁, 16 ₂ and to communicate and/or control the user interface20 to display or otherwise communicate the measured impedance to a user.The RF power source 22 may typically produce RF signals in the frequencyrange of between about 50 kHz and 500 kHz. The controller 18 may bestill further configured to control a switching function block 26 tomake the connections between the RF power source 22 and the plurality ofelectrodes contained in the electrode blocks 16 ₁, 16 ₂, in a mannerthat will become more apparent from the detailed description below. Thecontroller 18, user interface 20, RF power source 22, impedancedetection circuit 24 and switching function block 26 may each compriseconventional components known to those of ordinary skill in the art,except as may be otherwise noted herein. For example, the RF ablationgenerator 14 may comprise conventional apparatus, such as a commerciallyavailable unit sold under the model number IBI-1500T Cardiac AblationGenerator, available from Irvine Biomedical, Inc. Of course, the RFablation generator 14 can also comprise other known energy sources. Theart is replete with RF ablation generator configurations, designs,implementations and the like, and will therefore not be described in anyfurther detail.

Additional components (not shown) may also be integrated into the system10, such as visualization, mapping and navigation components known inthe art, including among others, for example, an EnSite™ ElectroAnatomical Mapping System commercially available from St. Jude Medical,Inc., and as also seen generally by reference to U.S. Pat. No. 7,263,397entitled “METHOD AND APPARATUS FOR CATHETER NAVIGATION AND LOCATION ANDMAPPING IN THE HEART” to Hauck et al., owned by the common assignee ofthe present invention, and hereby incorporated by reference in itsentirety. Additionally, an electrophysiological (EP) monitor or displaysuch as an electrogram signal display, or other systems conventional inthe art may also be integrated into the system 10. It should heunderstood that embodiments consistent with the present invention may,and typically will, include other features not shown or described hereinin FIG. 1 for the sake of brevity and clarity.

FIG. 2 is a flow chart showing steps of a method for two-stage RFablation according to an embodiment of the invention. The method beginsin step 28.

In step 28, the method involves conducting, during a first stage,bipolar ablation between a first pair of electrodes situated in anopposing arrangement on opposing sides of the tissue site so as to forma pair of opposing first stage ablation regions. This step may or maynot achieve complete transmural ablation (i.e., complete through thedepth of the tissue site), and accordingly may leave a channel ofun-ablated tissue between the first stage ablation regions. Since thetwo stage process can accommodate ablation that does not, during thefirst stage alone, achieve ablation through the complete depth of thetissue, the method contemplates that reduced power levels, reducedablation durations (e.g., time) or other variations may be used, whichhas the desirable result of avoiding overheating and yielding a reducedvolume of ablated tissue. The method proceeds to step 30.

In step 30, the method involves determining whether first stagetermination conditions have been met. This step may be expressed in anumber of different embodiments. In one embodiment, the user (e.g.,practitioner) may set an initial power level (e.g., constant power levelin watts) and duration, based on, for example, experience,patient/tissue site assessment and/or other factors known in the art.Accordingly, when the manually-set duration expires, the first stageablation is complete. Alternatively, the termination conditions mayinvolve assessment of the impedance between different pairs ofelectrodes, for example, when an impedance value is determined tosaturate (i.e., increase to a certain level or by a certain amount or bya certain factor and then substantially level off). This impedanceassessment may be made manually (i.e., by the user), or may beautomated, in which case the main controller 18 is configured to monitorthe impedance, as determined by the impedance detection circuit 24, andwhen the impedance saturation condition is detected, generating acontrol signal to the RF power source to automatically discontinue power(i.e., or otherwise discontinue application of power, such as bydisconnecting the power source via opening of the switching devices inthe switching function block 26, or in other ways known in the art).This impedance assessment feature will be described in greater detailbelow with respect to specific electrodes in specific embodiments. Inany event, if the termination conditions have not been met (“NO”), thenthe method branches back to step 28, where the first stage ablationcontinues. However, if the first stage termination conditions are met(“YES”), then the method proceeds to step 32.

In step 32, the method involves further bipolar ablation, during asecond stage, but now between a second pair of electrodes situated in adiametrical arrangement with respect to the first stage ablation regions(i.e., not the opposing arrangement described above). Because of theincreased impedance exhibited by the first stage ablation regions, thetissue that may remain unablated after the first stage (i.e., the regionbetween the first stage ablation regions) constitutes the proverbial“path of least resistance” for the ablation energy to be delivered andfocused into during this second stage, given the diametrical arrangementof electrodes. The second stage is thus operative to form a second stageablation region intermediate the opposing first stage ablation regions.The second stage ablation completes the ablation process, ensuring thatthe tissue site is ablated completely through its depth, withoutundesired, excessive volume of tissue being ablated.

FIG. 3 is a cross-sectional view of a biological tissue site 12 showingan RF ablation electrode configuration in a first embodiment. Asdescribed above, the ablation process is conceptually divided into twostages, and without loss of generality, such process will be describedfor specific embodiments involving bipolar ablation under the followingconditions: (1) the tissue-electrode interface is substantially flat;(2) the electrode width is small relative to its length (e.g.,length≧width; preferably, length≧2*width, more preferably,length≧3*width); (3) the heating intensity produced by the electrodeextends generally in a direction perpendicular to the electrode (whenviewing the cross-sectional profile, e.g., as in FIG. 3), and moreoverthat the cross-sections shown in the various Figures are removed by atleast several electrode-widths from the electrodes tips/forward edges;and (4) the RF electrodes that are not powered during a given ablationstage may be physically removed from tissue electrical contact oralternatively electrically isolated from the tissue, even though suchelectrodes may continue to be shown positioned on the tissue. With thisunderstanding, a number of embodiments will now be described.

With continued reference to FIG. 3, the upper and lower electrode blocks16 ₁ and 16 ₂ (e.g., as shown in FIG. 1 in block form) may be embodiedas a first pair of electrodes 34 (i.e., electrodes designated 1A and1B), a second pair of electrodes 36 (i.e., electrodes designated 2A+ and2B−) and a third pair of electrodes 38 (i.e., electrodes designated 2A−and 2B+). The first pair of electrodes 34 are situated in an opposingarrangement on opposing sides of the tissue site 12, while the secondand third pairs of electrodes 36, 38 are situated in first and seconddiametrical arrangements, respectively, i.e., as taken with respect tothe first pair of electrodes 34 or the ablation regions they create.During the first stage, only the first pair of electrodes 34 (i.e.,electrodes 1A and 1B) are powered in counter-phase by the RF ablationgenerator 14 for conducting bipolar ablation.

FIG. 4 shows one example of how two power signals 40, 42 may be producedin counter-phase. As illustrated, signals 40, 42 may be sinusoids, andfurther, may be offset in phase by 180 degrees (π radians). It should beappreciated that through this counter-phase configuration, electricalcurrent will alternately flow in both directions (i.e., from electrode1A→1B, and alternately 1B→1A). It should be further understood that thisconfiguration is exemplary only, and one of ordinary skill in the artwill recognize a wide variety of alternatives to accomplish bipolarablation between a pair of electrodes 34. The ablation generator 14,including the RF power source 22, is configured to produce RF powersignals in accordance with the particular phasing approach chosen (e.g.,here, a counter-phase approach). It should be further understood thatstill other variations are possible. For example, some unipolarcomponent may be employed during the first stage of ablation.

For such unipolar component embodiments, the electrode configurationdescribed herein would further include an indifferent or referenceelectrode, disposed either within the body or affixed to the bodysurface of the subject (i.e., skin patch electrode). The activation forthe unipolar component (first stage) may involve, in one embodiment,sequential activation of the electrodes 1A and 1B of the electrode pair34 (i.e., each electrode being vectored to the indifferent/referenceelectrode). Thus, while one electrode (e.g., electrode 1A) is activated,the opposing, non-activated electrode (e.g., electrode 1B) iselectrically isolated from the tissue site, such as, for example, byremoving the non-activated electrode from contact with the tissue site(i.e. spaced from the tissue—myocardium) or otherwise selectivelyelectrically insulating the electrode from the tissue. In anotherunipolar component embodiment, both electrodes are electrically drivenat the same time; however, not in counter-phase but rather with a phaseoffset greater than 0 degrees and less than 180 degrees (again, areference/indifferent electrode is used). It bears emphasizing that tothe extent unipolar ablation components are used, it would be usedduring the first stage ablation. The second stage ablation wouldcontinue to involve bipolar ablation.

FIG. 5 is a cross-sectional view (profile) of the tissue site 12 at theend of the first stage of ablation. Conducting bipolar ablation duringthe first stage forms a pair of opposing first stage ablation regions 44₁, 44 ₂ in the tissue at tissue site 12. At the end of the first stage,for a relatively thick tissue site 12, it is possible that the ablationdid not prowess all the way through the whole tissue depth, leaving anunablated region 46 in between the first stage ablation regions 44 ₁, 44₂. Further application of ablation power to the first pair of electrodes34 (i.e., individual electrodes 1A and 1B) can result in overheating ofthe already-ablated tissue near these electrodes, since the impedance ofthe ablated tissue is often significantly higher than the impedance ofthe non-ablated tissue (i.e., at least within a predetermined,relatively short time after ablation, while such tissue is at anelevated temperature).

FIG. 6 is a cross-sectional (profile) view of the tissue site 12during/after the second stage of ablation. To address the overheatingand potentially undesirable increased volume of ablated tissue, thefirst stage is discontinued (e.g., the electrodes 1A/1B are de-energizedand/or removed), and during the subsequent second stage, bipolarablation is conducted using the second and third pairs of electrodes 36,38. In the illustrated embodiment, bipolar ablation is conducted betweenelectrodes 2A+ and 2B+ (i.e., energized in a positive phase such as persignal 40 in FIG. 4) and electrodes 2A− and 2B− (i.e., energized in anegative phase such as per signal 42 in FIG. 4). The increase in theimpedance of the first stage ablation regions 44 ₁, 44 ₂ near electrodes1A/1B facilitates the ablation during the second stage, since thischaracteristic helps concentrate the RF current in and through thenarrow, unablated channel (region 46 in FIG. 5) between the first stageablation regions 44 ₁, 44 ₂. As a result, the tissue in and around theregion 46 (FIG. 5) is ablated during the second stage, forming a secondstage ablation region 48, without affecting significant tissue volumesin undesired regions.

In the embodiment of FIGS. 3 and 5-6, unnecessary ablation near theelectrodes 2A+/2A− and 2B+/2B− is suppressed through a number ofapproaches: (1) by configuring the electrodes 2A+/2A− and 2B+/2B− sothat they are wider than the electrodes 1A/1B; (2) by arranging theelectrodes 2A+/2A− and 2B+/2B− so that they are laterally offset fromthe first stage ablation regions 44 ₁, 44 ₂ by at least a predetermineddistance; and (3) by replacing each of the electrodes 2A+/2A− and2B+/2B− with properly driven multiple electrodes.

In addition, to better concentrate RF current between the first stageablation regions 44 ₁, 44 ₂, in one variation, the first pair ofelectrodes 34 (1A/1B) may be electrically isolated from the tissue site12 so that these electrodes (1A/1B) will not partially shunt current viathe first stage ablation regions 44 ₁, 44 ₂. Electrical isolation may beaccomplished by physically removing the first pair electrodes 34 fromelectrical contact with the tissue site 12 or alternatively interposinga relatively high impedance material, such as an insulative sheathmovable between a first conductive position and a second insulatingposition, between the first electrode pair 34 (1A/1B) and the RF powersource 22 of the ablation generator 14.

FIG. 7 is a timing diagram showing further embodiments that use onlyfour electrodes rather than three pairs (six electrodes) as used in thefirst embodiment while maintaining efficient ablation. In one furtherembodiment, during the first stage, designated as interval 50 on thetimeline, the first pair of electrodes 32 (electrodes 1A/1B) are used inthe manner described above. However, during the second stage ofablation, designated as interval 52, bipolar ablation may be conductedusing only the second pair of electrodes 36 (i.e., electrodes 2A+ and2B−). These electrodes may be positioned as in FIG. 3. A still furtherembodiment involves the concept of sub-dividing the second stageinterval 52. During a first sub-stage, designated as first interval 54,the second pair of electrodes 36 (i.e., electrodes 2A+/2B−) are situatedas shown in FIG. 3 and are then energized as described above to conductbipolar ablation. Then, during interval 56, the second pair 36(electrodes 2A+/2B−) are moved and are placed in the positions ofelectrodes 2A−/2B+ of FIG. 3, in essence in the same place as thenow-omitted third pair of electrodes 38. The electrodes 2A+/2B−(now-moved) are then energized as described above to conduct bipolarablation.

FIGS. 8-9 are cross-sectional views of the tissue site 12 showing astill further embodiment that involves four moveable electrodes. Beforethe first stage of ablation, electrodes 1A and 2A are electricallyconnected and are placed relatively close to each other and disposedapproximately opposite the electrodes 1B and 2B, which are themselveselectrically connected to each other. In effect, the two pairs ofelectrodes, taken together, are configured in the opposing arrangementdescribed above in connection for the electrode pair 34 (1A/1B) in theembodiment of FIGS. 3 and 5-6.

As shown in FIG. 8, during the first stage, bipolar ablation isperformed between the electrodes 1A/2A (i.e., both driven in a positivephase, such as per signal 40 in FIG. 4) and the electrodes 1B/2B (bothdriven in a negative phase, such as per signal 42 in FIG. 4). This firststage forms a pair of first stage ablation regions 44 ₁, 44 ₂ as shown.

FIG. 9 is a cross-sectional view showing the reconfiguration of theelectrodes, as compared to FIG. 8, for purposes of conducting the secondstage ablation. For the second stage, the two electrode pairs 1A/1B and2A/2B are moved away from their first location the opposing arrangement)to a second location where they are laterally spread apart, away fromthe first stage ablation regions 44 ₁, 44 ₂. Preferably, in the secondlocation, the electrodes have at least a predetermined minimum lateralspacing between the inner edges of the electrodes and the outer edges ofthe first stage ablation regions 44 ₁, 44 ₂. This new spacingconstitutes a dual diametrical arrangement, similar to that shown inFIG. 3 for the first embodiment. Next, the two pair of electrodes areelectrically reconfigured/reconnected in pairs 1A-1B and 2A-2B withthese pairs being driven in the opposite phases (e.g., electrode pair1A-1B being driven in a positive phase, such as per signal 40 in FIG. 4,while electrode pair 2A-2B is driven in a negative phase, such as persignal 42, or switching positive/negative phase orientation would alsowork). Energizing these reconfigured pairs results in the formation ofthe second stage ablation region 48, as shown. One advantage of thisembodiment is the reduced number of electrodes.

One challenge, however, is that this embodiment necessarily involvesmoving the electrodes and in addition, during the second stage, thetissue site 12 is no longer clamped across the center near the firstablation regions (i.e., where the two electrodes pairs were previouslylocated during the first stage). To address this challenge, in a stillfurther embodiment, an optional mechanical clamping device 60 comprisingclamp members 60 ₁ and 60 ₂ may be used, separable or partiallyseparable from the RF electrodes 1A/1B and 2A/2B. As shown, themechanical clamping device 60 takes the place vacated by the twoelectrode pairs after the first stage, thereby ensuring consistentcompression through the tissue site 12. This is particularly importantwhere the tissue layer includes multiple, individual layers, such as mayexist when conducting epicardial ablation (i.e., “pinching” or “folding”tissue, thereby lapping two individual layers).

In another embodiment, only two movable electrodes (and optionally aclamp) are used. For the first ablation stage, these electrodes are putin the central position (34 in FIG. 3). For the two substages of thesecond ablation stage, these electrodes are repositioned into positions36 and 38 (FIG. 3) for one and the other substages, respectively. Theoptional clamp could be placed in the middle of the tissue site (FIG.9), when the two electrodes are removed therefrom.

FIG. 10 is a side view of an apparatus 62 configured for use inperforming the ablation method described herein. The apparatus 62 mayinclude a pair of opposing substantially resilient elongate members 64,66 that couple the electrodes 1A/1B, 2A+/2A− and 2B+/2B− for RF energydelivery therebetween. The elongate members may comprise clampingmembers 64, 66 (as shown in FIG. 10) actuated via a pivot 68 in ascissors-like fashion.

FIG. 11 is a side view of apparatus 70 also configured for use inperforming the ablation method described herein. The apparatus 70 mayalso include a pair of opposing substantially resilient elongate membersdesignated 72, 74 configured so as to be slideably coupled so that onemember (e.g., member 72) is fixed and the other member (e.g., member 74)is moveable in direction 76 toward/away from the fixed member 72.

FIGS. 12-13 are partial top and side views, respectively, of a stillfurther 3-arm, scissors-type embodiment, designated apparatus 78. Theapparatus 78 includes a lower, clamping member 80, a first upperclamping member 82 and a second upper clamping member 84 comprised of apair of individual members 84 ₁ and 84 ₂. Handle portions 86, 88 and 90are used to control the motion of members 80, 82 and 84, respectively.The apparatus 78 includes a main pivot 92. The apparatus 78 provides themeans to separately move each clamping arm 80, 82 and 84 independent ofeach other. Thus, clamping arms 80, 82 can be moved into position sothat electrodes 1A/1B apply a clamping force to the tissue site 12during a first stage, while the clamping arm 84 can remain positioned sothat electrodes 2A+/2A− and 2B+/2B− do not contact the tissue site andthus do not exert a clamping force on the tissue. During the secondstage, the clamping arm 84 can be moved into position so that electrodes2A+/2A− and 2B+/2B− contact the tissue site and thus exert a clampingforce to the tissue. In the embodiments of FIGS. 10-13, suitableelongate conductors, switches couple the electrodes to the RF generator14. Thus, generally, the apparatus 78 is configured such that theelongate members comprise at least two pairs of opposing members so thatboth pairs can engage a target tissue site in a compressive state and atleast one can be relaxed in a non-compressive state thereby not engagingthe target tissue site. In one form the members of the embodimentsdescribed herein are designed and fabricated of biocompatible materialand of dimension providing ease of pericardial access via, for example,a sub-xiophoid incision to access one or more pulmonary veins of asubject.

The ablation process may be effectively monitored by checking theimpedance between one or more different pairs of electrodes. Taking thefirst embodiment (i.e., FIGS. 3 and 5-6) as exemplary, the first stageablation may be terminated (“first stage termination conditions”) whenthe impedance between electrode 1A and electrode 1B approximatelysaturates (i.e., when, after increasing initially after ablation starts,levels off to an approximately constant or level value). During thesecond stage, the impedance between a number of electrode pairs may bemonitored, for example: (1) between electrodes 2A+ and 2B+ (i.e., toestimate if any unwanted RF ablation occurs near these electrodes, whichwould reveal itself by an increase in impedance value); (2) betweenelectrodes 2A− and 2B− (i.e., also to estimate if any unwanted RFablation occurs near these electrodes); (3) between electrodes 2A+ and2B−, the diametrical arrangement, to estimate if the second stageablation succeeded (e.g., forming a satisfactory second stage ablationregion 48); and/or (4) between electrodes 2A− and 2B+, the diametricalarrangement, also to estimate if the second stage ablation succeeded(e.g., forming a satisfactory second stage ablation region 48). Thelatter two situations may be monitored, for example, to ascertainimpedance saturation as described above, and used to determine when toterminate the second stage (“second stage termination conditions”). Itshould be understood, as alluded to above, that the impedance monitoringmay be performed manually (i.e., as observed by the user), or,alternatively, may be implemented in automated monitoring/automaticstart/stop of the ablation stages. In the automated embodiment, the maincontroller 18 of the ablation generator 14 may be configured in softwareto implement the above steps, consistent with the description herein.

It should be understood that the system 10, particularly the RF ablationgenerator 14, as described above may include conventional processingapparatus known in the art, capable of executing pre-programmedinstructions stored in an associated memory, all performing inaccordance with the functionality described herein. Certain aspects ofthe methods described herein may be programmed (as indicated), with theresulting software being stored in an associated memory and where sodescribed, may also constitute the means for performing such methods.Implementation in software, in view of the foregoing enablingdescription, would require no more than routine application ofprogramming skills by one of ordinary skill in the art. Such a systemmay further be of the type having both ROM, RAM, a combination ofnon-volatile and volatile (modifiable) memory so that the software canbe stored and yet allow storage and processing of dynamically produceddata and/or signals.

In addition, the ablation electrodes described herein may be coupled toonly an RF generator or one or more electrodes can also couple to acardiac pacing circuit and/or a cardiac sensing circuit or impedancemeasuring circuit (as described above) according to various forms of theinvention. The pacing and/or sensing circuitry can be used to confirmtransmurality of a particular lesion set (e.g., a continuous transmurallesion around a plurality of pulmonary veins) whether or not the set wasproduced by a system according to the foregoing or in part by anotherapparatus. That is, pacing or sensing “above” the lesion set (abovenatural sinus rhythm) should not conduct or capture the myocardium thusindicating continuity of the lesion set. Likewise, sensing above thelesion set should not detect cardiac activity (or at least not attypical amplitude(s)).

Although numerous embodiments of this invention have been describedabove with a certain degree of particularity, those skilled in the artcould make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this invention. All directionalreferences (e.g., plus, minus, upper, lower, upward, downward, left,right, leftward, rightward, top, bottom, above, below, vertical,horizontal, clockwise, and counterclockwise) are only used foridentification purposes to aid the reader's understanding of the presentinvention, and do not create limitations, particularly as to theposition, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other. It is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative only and not limiting.Changes in detail or structure may be made without departing from thespirit of the invention as defined in the appended claims.

1.-21. (canceled)
 22. An apparatus, comprising: means for conductingbipolar ablation between a first pair of electrodes in an opposingarrangement on opposing sides of the tissue site so as to form a pair ofopposing first stage ablation regions in the site during a first stage;and means for conducting bipolar ablation between a second pair ofelectrodes in a diametrical arrangement with respect to the first stageablation regions to form a second stage ablation region intermediatesaid first stage ablation regions during a second stage.
 23. Anapparatus according to claim 22, wherein the diametrical arrangementcomprises a first diametrical arrangement, and further comprising: meansfor conducting bipolar ablation during the second stage between a thirdpair of electrodes in a second diametrical arrangement with respect tothe first stage ablation regions.
 24. An apparatus according to claim22, wherein the second pair of electrodes comprises a different pairfrom the third pair of electrodes.
 25. An apparatus according to claim22, wherein the second stage comprises a first interval and a secondinterval and wherein said means for conducting bipolar ablation duringthe second stage further comprises: means for conducting bipolarablation during the first interval using the second pair of electrodesin the diametrical arrangement; means for moving the second pair ofelectrodes to a second diametrical arrangement; and means for conductingbipolar ablation during the second interval using the second pair ofelectrodes in the second diametrical arrangement.
 26. The apparatus ofclaim 22 wherein said pair of opposing first stage ablation regions inthe site extend from respective sides of said tissue site towards acenter of said tissue site, said first stage being conducted inaccordance with at least one of a selected power and time durationparameter such that upon termination of said first stage ablation, saidpair of opposing first stage ablation regions together extend less thansaid depth of said tissue site; and wherein said second stage ablationregion is in the center of said tissue site and is different than saidfirst stage ablation regions wherein said second stage ablation regionscompletes the ablation through the entire depth of said tissue site. 27.The apparatus of claim 26 wherein said second stage occurs aftertermination of said first stage.
 28. The apparatus of claim 26 whereinsaid means for conducting bipolar ablation between said first pair ofelectrodes and said means for conducting bipolar ablation between saidsecond pair of electrodes, comprises a radio frequency (RF) ablationgenerator including a main controller and an RF power source selectivelycoupled to said first and second pairs of electrodes, said maincontroller being configured to control the operation of said RF powersource.
 29. The apparatus of claim 28 wherein said main controller isconfigured to control operation of said RF power source in accordancewith a plurality of inputs received from a user thereof through a userinterface.
 30. The apparatus of claim 28 wherein said main controller isconfigured to interact with an impedance detection circuit to obtaindata indicative of an impedance value measured through or using one ormore of the electrodes.
 31. The apparatus of claim 28 wherein said maincontroller is further configured to control a switching function blockto make connections between said RF power source and said first andsecond pair of electrodes.
 32. The apparatus of claim 28 wherein saidmain controller is configured to control said RF power source so as toprovide power in counter-phase to the electrodes of at least one of saidfirst and second pairs of electrodes.
 33. The apparatus of claim 32wherein said main controller is further configured to control said RFpower source so as to apply a first signal to one of the electrodes ofsaid at least one first and second electrode pairs and to apply a secondsignal to the other one of the electrodes of said at least one first andsecond electrode pairs, wherein said first and second signals are offsetin phase by 180 electrical degrees.
 34. The apparatus of claim 33wherein said first and second signals are alternating current (AC)signals having a predetermined frequency.
 35. The apparatus of claim 34wherein said predetermined frequency is in the radio frequency (RF)range between about 50 kHz and 500 kHz.
 36. The apparatus of claim 28wherein said RF ablation generator is configured to power said firstpair of electrodes at a substantially constant power.
 37. The apparatusof claim 28 wherein said main controller is configured to monitor animpedance of said tissue site as detected by an impedance detectioncircuit and discontinue bipolar ablation during said first stage whensaid monitored impedance satisfies predetermined criteria.
 38. Theapparatus of claim 37 wherein said predetermined criteria includes asaturation condition.
 39. An apparatus, comprising: a main controller;and an RF power source configured to be coupled to a first pair ofelectrodes and to a second pair of electrodes; wherein said maincontroller is configured to control the operation of said RF powersource for conducting bipolar ablation between said first pair ofelectrodes in an opposing arrangement on opposing sides of a tissue siteso as to form a pair of opposing first stage ablation regions in saidsite during a first stage and for conducting bipolar ablation betweensaid second pair of electrodes in a diametrical arrangement with respectto said first stage ablation regions to form a second stage ablationregion intermediate said first stage ablation regions during a secondstage.
 40. The apparatus of claim 39 wherein said pair of opposing firststage ablation regions in said site extend from respective sides of saidtissue site towards a center of said tissue site, said main controllerbeing configured to conduct said first stage in accordance with at leastone of a selected power and time duration parameter such that upontermination of said first stage ablation, said pair of opposing firststage ablation regions together extend less than said depth of saidtissue site; and wherein said second stage ablation region is in thecenter of said tissue site and is different than said first stageablation regions wherein said second stage ablation region completes theablation through the entire depth of said tissue site.
 41. The apparatusof claim 40 wherein said main controller is further configured tocontrol a switching function block to make connections between said RFpower source and said first and said second pair of electrodes, saidmain controller is further configured to control operation of said RFpower source in accordance with a plurality of inputs received from auser thereof through a user interface, and wherein said main controlleris further configured to monitor an impedance of said tissue site asdetected by an impedance detection circuit and discontinue bipolarablation during said first stage when said monitored impedance satisfiespredetermined criteria comprising saturation.